The invention relates to ultrasonic processes according to the introductory clause of claims 1 or 2 and circuits for performing them.
In ultrasonic technology, ultrasonic waves are irradiated in an examination area for selective graphic representation and/or evaluation of the Doppler spectrum. Combined transceiver sound heads are usually used in the processes and equipment for material testing and for examination of biological tissues. In this way, a sound frequency (fo), which is the same for the sending and receiving, is determined by the crystals of the oscillator and the equipment electronics. A typical 5 MHz sound head has a frequency range of about 3-7 MHz with a maximum at fo=5 MHz. In the same frequency range, the reflected and/or backscattered signal is received with the pulse-echo process. Such equipment and processes are also used in the examination of biological tissue with the use of ultrasonic contrast media. Signal portions lying outside the specified frequency range, such as, for example, oscillations being in a harmonic ratio to the sending frequency, are not used for the graphic representation of the examination object and other analyses, such as, e.g., Doppler measurements. In the previously known processes and equipment systems, several sound heads, which are changed during the examination, are further used to cover a sizable frequency range.
Improving the image quality by using harmonic multiples of the excitation frequency in ultrasonic microscopy is known from the bibliographical reference L. Germain, J. O. N. Cheeke, (J. Accoust. Soc. Am. E3 (1988) 942). For this purpose, however, ultrasonic waves with very high amplitude have to be irradiated to produce nonlinear oscillations in the path in the examination area, and by this nonlinearity, a transmission of energy from the oscillations with the basic frequency takes place in higher harmonic oscillations.
But such a process cannot be used in the ultrasonic examination with low frequencies, for example, in the range of 1-10 mHz of objects which are not resistant to high sound intensities, such as, especially, biological tissues.
The object of the invention is to broaden the range of use of ultrasonic processes for objects, limitedly resistant to sound intensity, especially biological tissues, for selective graphic representation and evaluation of the Doppler spectrum and to provide circuits for performing these processes.
According to the invention, this object is achieved in that a material is introduced in the examination area to be acoustically irradiated, with which nonlinear oscillations in this area are produced by irradiated ultrasonic waves, a broad-band, acoustically highly damped, electrically matched ultrasonic converter with one or more controllable converter elements assembled individually or in groups, which corresponds to a frequency band which, in addition to the excitation frequency, comprises at least {fraction (xcex1/2)} and/or {fraction (xcex1/3)} and/or {fraction (xcex1/4)} times excitation frequency (fo), with xcex1=whole number, is excited for acoustic irradiations of the examination area and the excitation frequency and/or at least one of {fraction (xcex1/2)}, {fraction (xcex1/3)}, {fraction (xcex1/4)} times it are evaluated from the ultrasonic signal received by the ultrasonic converter, reflected from the examination area or backscattered from the latter.
If xcex1 is an even-numbered multiple of the denominator, the corresponding oscillations are the harmonics. If xcex1 less than  the denominator, these oscillations are called subharmonics in the literature. If xcex1 greater than  the denominator, ultraharmonic oscillations are involved.
By introducing materials or media in the examination area to be acoustically irradiated, which produce a nonlinearity, it is surprisingly possible, even in low sonic intensities, which are not harmful, to obtain intensive and strongly frequency-shifted stray and/or transmission signals in addition to excitation frequency for These stray and/or transmission signals are intensive particularly in harmonics (2 fo, 3 fo . . . ), subharmonics (xc2xd fo, ⅓ fo, xc2xe fo) and ultraharmonics ({fraction (3/2)} fo, {fraction (5/4)} fo . . . ) of the excitation frequency. With this process,irradiation can be performed with low frequencies, so that a greater penetration depth is obtained and receiving signals of higher frequencies can be evaluated.
In an advantageous way, a selective evaluation of the signal portions influenced by the fed materials or media as well as a selective display of the areas filled with these media is possible, without a previously necessary subtraction being made between two or more states recorded before and after the application of the materials or media. Especially, the Doppler effect caused can be evaluated independently of artifacts.
Nonlinear stray elements are advantageously introduced in the examination area. But in the examination area, a nonlinear ultrasonic contrast medium can also be introduced in the form of a solution or suspension and especially microbubbles or agents producing microbubbles.
The introduction of a microbubble suspension with a concentration of 10xe2x88x923% by weight to 30% by weight of dry substance in a suspension medium leads to good results, and, surprisingly, the low lower limit of 10xe2x88x923% by weight is attained with the process according to the invention and the circuit according to the invention.
In the process according to the invention, the sonic converter is advantageously excited by a function generator, with which HF bursts with adjustable amplitude and adjustable center frequency (fT) are produced in the range of 0.3 MHz to 22 MHz, preferably 1 MHz to 11 MHz, and with 0.5 to 20, preferably 1-5, periods. In this case, it has been shown as especially advantageous to evaluate frequencies which are smaller than sonic converter (transmitter) center frequency fT.
In the evaluation, it is advantageous, by a computer-controlled gate circuit, to select at least one period and to determine the related frequency spectrum in an analog or digital manner. In this case, the time window length and the number of periods per burst are adjusted between the optimum frequency resolution and optimum high-sensitivity resolution.
With the process according to the invention, Doppler effects can advantageously be evaluated in harmonics of the excitation frequency and in the mixed products, such as, the upper sideband in 2-frequency excitations. This permits the representation of slower flows without disturbances by movements of the vessel wall.
An improved penetration depth and/or space resolution, which is very advantageous in graphic representation and in Doppler measurements, further results in the evaluation of harmonic signal portions or signals in the upper sideband.
The circuit, according to the invention, for performing the process described above exhibits a function generator whose output is connected with the oscillator of an acoustically highly damped, electrically matched, broadband converter element by an T/R (transceiver) circuit synchronized by the function generator, which is downstream from a signal processing system.
In another embodiment of the circuit, the function generator is connected to the input of a converter whose output is connected to a signal processing system.
In the first case mentioned, the burst produced by the function generator in the xe2x80x9csendingxe2x80x9d circuit of the T/R switch is transmitted to the oscillator of the converter and the signal received by the converter is transmitted in the switched-on xe2x80x9creceivingxe2x80x9d position of the T/R circuit to the evaluating system. In the second case, the input and output are separated in the converter so that an T/R switch is not necessary.
A converter element, whose center frequency fT is greater than the upper limit of the operating range, is used with special advantage. This converter element is designed so that, as a function of the frequency in the frequency range below excitation or center frequency fT, the sonic intensity radiated by the converter element exhibits a positive first derivative with respect to the frequency, which is approximately constant especially in the operating range, or so that the sonic intensity which is in the operating range itself has a constant value. By this almost rectilinear frequency response in the operating range for a similar frequency response, especially the damping can be largely balanced, in the irradiated examination area. By this circuit and the converter used, a change of the frequency used for examination is possible without a change in sound heads. Further, in the evaluation of spectra for material characterization, especially in the tissue characterization, the respectively optimum ratio of space resolution and frequency resolution can be selected.
The process according to the invention can advantageously be performed by a circuit, which exhibits a multielement converter with phase-delayed, actuated converter elements, to perform a phase-array or a dynamically focused process. In this circuit, the output of a function generator is connected by an n-path signal divider, n computer-controlled delay circuits and n T/R switches, controlled by the function generator or a computer, to the inputs of n acoustically highly damped, electrically matched, broadband converter elements, whose outputs are connected by n T/R switches each to an m-path signal divider. These m-path signal dividers are each connected by m delay circuits and m fixed or variable circuits for frequency band selection and further by a circuit for in-phase summation and optionally signal distribution to a system for selective further processing of m frequency bands.
In another achievement of the object of the invention, a material is introduced in the examination area to be acoustically irradiated with which nonlinear oscillations are produced in this area by irradiated ultrasonic waves, a broadband, acoustically highly damped, electrically matched ultrasonic converter with one or more controllable converter elements individually or assembled in groups is excited by two HF bursts, whose respective excitation frequencies are different from one another, and are smaller than half the frequency upper limit of the operating range, and signal combinations of both excitation frequencies, especially their sum or difference frequency, are evaluated from the ultrasonic signals received from the ultrasonic converter, reflected from the examination area or backscattered from the latter.
In this case, a stronger receiving signal with a frequency of the combination of the frequencies of the irradiated signals, especially the sum or difference frequency, is obtained by irradiation of two signals separated from one another. In this case, the sum frequency is especially of interest because of the achievably higher space resolution. In this process, a converter element can be excited by two HF bursts. But the possibility also exists of exciting two separate converter elements. respectively with one HF burst, and the center frequencies of these HF bursts vary and are smaller than half the upper limit of the frequency of the operating range.
By the nonlinearity produced according to the invention, a stronger receiving signal at fo+fp, i.e., at about 4 MHz, is obtained, for example, with two low-frequency signals, e.g., fo≈fp≈2 MHz, than if only one sending signal of frequencies fo+fp is used with same total output Io, Ip. This phenomenon makes possible a higher penetration depth at high observation frequencies.
As materials or media, which produce the nonlinearity, the same can be used as in the process for evaluating the harmonic frequencies of the excitation frequency. Basically, the same circuit elements can be used with addition of a second HF generator.
In the circuit with a multielement converter, the second signal is always sent only in the respective direction of the first signal to reduce the average output irradiated in the examination area and is begun about 1 to 2 periods earlier and lasts until the end of the first burst signal. To achieve this, the second signal from the second generator is influenced by corresponding delay circuits so that it reaches the same converter elements in the sound head after passage of the T/R. switch and is radiated in the same direction as the first sending signal. The circuit matrix then receives signals in the frequency sum. In this case, the T/R switch is controlled by the longer-lasting second sending signal.